Abstract We have developed an alternative laser deposition technique for preparation of droplet-free thin films and fine particles. This separated pulsed laser deposition (SPLD) consists of an ablation chamber and a deposition chamber which are independently evacuated under different ambient gases. High quality ZnO films exhibiting both particle-free and uniform deposition were obtained such as at an ablation pressure of 5 mTorr (Ar) and a deposition pressure of 10 mTorr (O 2 ). It is shown that, when the bias voltage and magnetic field are simultaneously applied in the deposition chamber, the E × B drift motion of about 70 km/s over the substrates contributes to deposit the smooth and high quality films. XRD and optical transmittance for the deposited films were investigated to clarify the operating properties of the SPLD assisted by the electric field and magnetic field. This SPLD is a promising technique for preparation of films and nanostructured particles

Kenji Ebihara

Koji Fujii

Sang-Moo Park

Tomoaki Ikegami

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ls 15 (2006) 989–993
www.elsevier.com/locate/diamondDiamond & Related MateriaSeparated pulsed laser deposition for nanostructured thin films
Kenji Ebihara ⁎, Sang-Moo Park, Koji Fujii, Tomoaki Ikegami
Department of Electrical and Computer Engineering, Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1,
Kumamoto City, 860-8555 Japan
Available online 17 February 2006Abstract
We have developed an alternative laser deposition technique for preparation of droplet-free thin films and fine particles. This separated pulsed
laser deposition (SPLD) consists of an ablation chamber and a deposition chamber which are independently evacuated under different ambient
gases.
High quality ZnO films exhibiting both particle-free and uniform deposition were obtained such as at an ablation pressure of 5 mTorr (Ar) and
a deposition pressure of 10 mTorr (O2). It is shown that, when the bias voltage and magnetic field are simultaneously applied in the deposition
chamber, the E×B drift motion of about 70 km/s over the substrates contributes to deposit the smooth and high quality films. XRD and optical
transmittance for the deposited films were investigated to clarify the operating properties of the SPLD assisted by the electric field and magnetic
field. This SPLD is a promising technique for preparation of films and nanostructured particles.
© 2006 Elsevier B.V. All rights reserved.Keywords: Pulsed laser deposition (PLD); Nanostructures; Oxidation; Optical properties characteristics1. Introduction
Pulsed laser deposition (PLD) is a widespread and useful
technique possessing many advantages including stoichiometry,
lowering deposition temperature and high deposition rate in
arbitrary atmosphere [1]. However, the PLD has inherent
problems such as deposition of droplets, limited deposition area,
and expensive instrument system. Several novel systems
assisted by plasma, electric and magnetic fields have been
proposed to improve the deposited film quality. Giardini et al.
[2] and Cappelli et al. [3] used reactive pulsed laser ablation and
deposition technique which combines the several advantages of
conventional PLD with the enhancement of the reactivity in RF
plasma. Kobayashi et al. have developed Aurora PLD under
application of the magnetic field (0.45 T). This method is
characterized by an enhanced ionization of the ablated particles
during transport from the target to substrate through magnetic
influence on electrons and ions [4]. Zhang et al. utilized an
excimer UV source (Xe2⁎, 172 nm excimer lamp) in con-
junction with the conventional PLD in order to assist the growth
of oxide dielectric films in oxygen radicals [5].⁎ Corresponding author. Tel.: +81 96 342 3628; fax: +81 96 342 3630.
E-mail address: ebihara@eecs.kumamoto-u.ac.jp (K. Ebihara).
0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.diamond.2005.12.041We have developed an alternative laser deposition technique
to improve the film qualities by the conventional pulsed laser
ablation system [6]. Here, we propose the separated pulsed laser
deposition (SPLD) consisting of an ablation chamber and a
deposition chamber which are independently evacuated under
different ambient gases. In order to prepare the high quality PLD
films, electric and magnetic fields were applied in the deposition
chamber. Photonic material of ZnO was deposited to clarify the
features of this SPLD system.
2. Experimental
Fig. 1 shows the geometry of the SPLD system [6]. We can
independently control the ambient pressures of the ablation
chamber and the deposition chamber. The ablation chamber is a
stainless steel globe with 300 mm diameter. The deposition
chamber is made of a quartz tube of 100 mm diameter and a
metallic conic wall with variable orifice diameters. For the
ablation process we use a KrF excimer laser with λ=248 nm and
25 ns pulse duration. Different gas pressures in two chambers
having the differential evacuation system allow us to provide
optimal control of the plasma plume, the gas phase reaction and
the film growth by applying variable voltages between the conic
wall and the substrate under the magnetic field.
Fig. 1. Separated pulsed laser deposition.
990 K. Ebihara et al. / Diamond & Related Materials 15 (2006) 989–993Fig. 2 is the configuration of the components for the SPLD.
The magnetic induction (B) is generated by the permanent
magnets in the deposition chamber. The electric field (E) was
directed toward the left-hand side due to applied positive bias
voltage between the substrate and the conical capsule. Since
the radial component of the magnetic induction (Br) crosses
vertically with E, the drift motion of ions and electrons given
by
Vd ¼ E  B=B2 ðm=sÞ
provides the velocity Vd=E /Br [7].
This azimuthal averaged motion causes the plasma to flow
by sweeping over the deposited film.
We studied the dynamics of the plasma plumes of this SPLD
assisted by the applied electric and magnetic fields. The surface
morphology of the deposited films was observed by using
atomic force microscopy (AFM) (Seiko: SPI3800N).3. Results and discussion
In order to evaluate the operation condition of this SPLD
system, we have studied the dynamics of the plasma plume as
well as the film qualities of the deposited films. The plasmaMagnetic field B
Electric field E
Br
Vd
Substrate
Magnet
Vb
+
+
+ _
_
_
Fig. 2. E×B drift motion in the SPLD.plumes of the zinc oxide (ZnO) target in Ar and O2 atmosphere
of the ablation chamber (Pa=20 mTorr) were studied when the
deposition chamber was filled with oxygen gas of deposition
pressure (Pd=10 mTorr). It was noticed that the emission from
the argon plasma in the ablation chamber is strong and large
volume comparing with the plasma in oxygen atmosphere.
Since excitation and ionization of Ar gas are easier than these of
oxygen, the Ar plasma plume can enhance the ionization and
reaction of the ejected species.
In order to visualize and digitally record the plasma emission
spectra a linear CCD Ocean Optics spectrometer was used. The
emission spectra from the plasma were measured at the metallic
conic wall orifice with the help of a fibre optic and a collimator
attached to the spectrometer entrance slit. The time integrated
spectral intensity of the laser produced plasma at Pa=10 mTorr
O2 and Pb=20 mTorr O2 showed three strong lines from Zn
(468.0, 472.2, 468.8 nm) and a band from O2
+ at 637 nm.
The effect of magnetic field on the plasma plumes is
shown in Fig. 3. Plasma plume showed large emission vol-
ume in the ablation chamber when the mirror type magnetic
field (B=0.1 T) was applied. The emission in the deposition
chamber was too weak to measure using our optical system. The
magnetic field was extended to trap the charged particles near the
capsule type wall.
We deposited the ZnO films when pressures in both ablation
chamber and deposition chamber were changed. Fig. 4 shows
AFM images of the films deposited at Pa=5, 10, 20 mTorr
(Ar) under a deposition pressure (Pd) of 10 mTorr (O2). We
cannot observe droplet-like particles on the films. The small
grain of the size of 60 nm was obtained at lower pressure of Pa=
5 mTorr.
Fig. 5 shows the transmittance spectra of these films. The
film deposited at 5 mTorr exhibits higher optical transmittance
above 350 nm. The Tauc plot of this film indicates the optical
band gap of 3.1 eV. Increase of the ambient pressure decreases
optical band gap to about 2.6 eV.
Fig. 6 shows the XRD patterns of same ZnO films in Fig. 4.
The film deposited at Pa=5 mTorr and Pd=10 mTorr exhibits
the crystal structure with high preferential orientation of (002)
plane indicating good crystallinity.
Correlation between grain size and optical properties has
been discussed from a viewpoint of the band gap energy of the
deposited ZnO films. The existence of the grain boundary
causes a band bending to reduce the band gap energy becausePa=5mTorr(Ar), Pd=10mTorr(O2), Laser fluence: 3J/cm
2
target
orifice
B=0.1TB=0
Fig. 3. Effect of magnetic field on the emission of SPLD plasma.
RMS 14.46nm
Grain Size 149nm
Pa:Ar, 20 mTorr
Pd:O2, 10 mTorr
RMS 9.65nm
Grain Size 68nm
Pa:Ar, 10 mTorr
Pd:O2, 10 mTorr
RMS 5.2nm
Grain Size 61nm
Pa:Ar, 5 mTorr
Pd:O2, 10 mTorr
(a) (b) (c)
200
400
600
800
0
200 400 600 8000
Fig. 4. AFM images of ZnO films deposited at various values of Pa. Bias voltage=500 V DC.
0
20
40
60
80
100
200 300 400 500 600 700 800
T
 (
%
)
wavelength (nm)
(a)
(b)
(c)
(a) Pa = 20 mTorr
(b) Pa = 10 mTorr
(c) Pa =  5  mTorr
3.10 eV(c)
2.58 eV(b)
2.67 eV(a)
bandgap
Fig. 5. Transmittance of ZnO films deposited at various ablation pressures.
20 30 40 50 60 70 80
In
te
ns
it
y 
(a
rb
.u
ni
ts
)
2θ (degree)
(c) Ar =
Ar =
 Ar =
5 mTorr
(b)  10mTorr
(a)  20mTorr
Fig. 6. XRD patterns of ZnO films at various Ar pressures.
991K. Ebihara et al. / Diamond & Related Materials 15 (2006) 989–993the residual strain in the ZnO film causes the bandgap to shrink.
The change of the bandgap corresponding to the strain is very
small value in the range of 20–40 meV [8]. The remarkable
difference in the optical properties of the present results
(Fig. 5) is considered to be due to crystallinity. The sample (c)
consisting of small grains is highly crystallized as shown in
Fig. 6c. On the other hand the sample (a) seems to be non-
crystalline (amorphous) from the XRD pattern (Fig. 6a). At low
ablation pressure, the large volume plasma of ions, electrons,
radicals, atoms and molecules having low density with high
energy is generated to arrive on the substrate through the
reaction in the deposition chamber. The rarefied ambient gas
near 5 mTorr in the ablation chamber is found to contribute to
formation of small crystallized grain. As the ambient pressure
increases, the number of the particles and their energy (velocity)
arriving on the substrate decrease due to increasing collision
with ambient gas, resulting in deterioration of the deposited
films.0
20
40
60
80
100
200 300 400 500 600 700 800
T
 (
%
)
wavelength (nm)
(a)500V+0.1T 
(b)200V+0.1T
(c)100V+0.1T
(d)0V +0.1T  
2.98 eV(c)
2.75 eV(d)
2.55 eV(b)
2.80 eV(a)
bandgap
Fig. 7. Effect of electric field on trasmittance of ZnO films at Bs=0.1 T.
E(70V/cm)
RMS 5.2 nm
Grain Size 61nm
B( 0.1 T )
RMS 2.6 nm
Grain Size 50nm
RMS 0.84 nm
Grain Size 41nm
E(70V/cm) + B(0.1T)(a) (b) (c)
200
400
600
800
0
200 400 600 8000
Fig. 8. AFM images of ZnO films deposited at magnetic field and electric field. Pa=5 mTorr Ar, Pd=10 mTorr O2.
992 K. Ebihara et al. / Diamond & Related Materials 15 (2006) 989–993We studied the effect of the DC bias voltage and magnetic
field on the film properties. Fig. 7 shows the transmittance of
the ZnO films deposited at various bias voltages under a fixed
magnetic field (B=0.1 T). The 500 V films show the optical
band gap of 2.8 eV. The effect of the electric field and the
magnetic field on the surface morphology of the ZnO films is
shown in Fig. 8.
The charged particles (ion and electron) in the plume plasma
are affected by the electric and magnetic fields. The laser ablated
plasma was generated in the magnetic field where the electric
field was simultaneously applied to the substrate. The DC
voltage was applied between the substrate and the capsule se-
parated by 7 cm. In these fields, the E×B drift motion can realize
the enhancement of gas ionization and the control of film
growth. It is shown that the film deposited at 0.1 T and 500 V
(equivalent to E=70 V/cm) has smooth surface with RMS
0.84 nm. In this case, the azimuthal drift velocity Vd is estimated
to be 70 km/sec from the Eq. (1). This sample consists of small
grains with 40 nm. The XRD patterns for these samples show
that the FWHM of the film at 500 V has small value of 0.318°
indicating good crystallinity.
The charged particles moving with the azimuthal (parallel to
the substrate surface) motion can sweep the deposited surface of
the substrate which increases the smoothness of the films.
Although the electric field forces to accelerate the charged
particles (electrons at positive DC bias) toward the substrate
resulting in enhancing the crystallization, the magnetic field can
trap the charged particles to increase number density near the
reaction area in the deposition chamber. The velocity component
normal to the substrate will decrease and, as the substitution of
this velocity reduction, the azimuthal drift appears resulting in
decreasing the energy of the particles impinging on the substrate.
In conclusion, magnetic field superposed on the electric field
contributes to the smooth and uniform film deposition by
suppressing the crystal growth. This is the fact that the sample(c) in Fig. 7 (B=0.1 T,E=70V/cm) does not show the noticeable
improvement in the optical property. When the bias voltage
decreases, the surface roughness increases due to decrease of the
E×B drift effect.
4. Conclusion
The separated pulsed laser deposition (SPLD) was
proposed to improve the deposition characteristics by the
conventional PLD. It was shown that high quality ZnO films
exhibiting both particle-free and uniform deposition are
obtained at ablation pressure of 5 mTorr (Ar) and deposition
pressure of 10 mTorr (O2). The simultaneous application of
electric filed and magnetic field for the SPLD contributes to
prepare the crystalline films with sooth surface and small grain
size.
This SPLD is useful to prepare oxide and nitride thin films
and nanostructured materials including carbon nanotube and
nanoparticles by controlling the atmosphere in the separated
deposition chamber.
Acknowledgments
This work is supported in part by a Grant-in-Aid Scientific
Research from the Ministry of Education, Science, Sports and
Culture (No. 10045046).References
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[5] Jun-Ying Zhang, Ian W. Boyd, Applied Surface Science 186 (2002) 40.
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Separated pulsed laser deposition for nanostructured thin films

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Separated pulsed laser deposition for nanostructured thin films

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